Mechanistic studies of teratogens often require some information on the distribution of the toxic agent and its metabolites. A number of basic questions about methylmercury' mechanisms of action might be answered if its microscopic distribution where known, but every attempt to visualize the compound has met with serious criticisms that have left teratologists with no validated method for localization of this potent neurteratogen. New data demonstrate that methylmercury (along with all other forms of Hg) can be localized by the use of 203Hg with emulsion autoradiography (Rodier and Kates, 1986) and that inorganic Hg can be localized by the use of a silver sulphide technique (Magos et al., 1985). Thus, we propose a double labeling technique which will allow localization of the active organic form and will follow its conversion to the less active inorganic form. These will be examined in a variety of cell types and treatment conditions which are known or thought to differ in their response to MeHg. These include 1) mature vs developing cerebral cortex. 2) Male vs female neonatal cerebellar cortex 3) proliferation vs post mitotic cells 4) cells protected or not protected by the blood-brain barrier 5) cells exposed to MeHg directly vs those exposed systemically. Whether or not the distribution of organic and inorganic Hg is correlated with injury in the situations under study, the results will provide the first detailed descriptions of Hg distribution in the CNS - a necessary starting point for other studies.